Geographically dispersed transceiver sites are often employed in two-way radio communication systems to support subscribers operating over a wide coverage area. Ordinarily, the sites are arranged such that each provides coverage for a particular geographic region, which may have some overlap. A subscriber interfaces with the radio communication system through a particular transceiver site, referred to herein as a server site. When a subscriber moves from one location to another, it may be advantageous to switch server operations from one site to another, such as a neighboring site, in order to maintain a high quality communication link. The process of switching server sites during an ongoing communication session is generally referred to in the art as hand-off or hand-over. The hand-off decision process usually involves a determination of the quality of signals received by the subscriber from its server site, or vice versa, and the availability and suitability of other potential server sites that may be in communication range of the subscriber. The hand-off decision may be made using information gathered by the server site, information gathered by the subscriber, or a combination of both.
In one prior art example, a subscriber maintains a list of control frequencies used by neighboring sites, and monitors the signal quality and/or other communication parameters for signals emanating from these sites. The information gathered by the subscriber is then used to select a suitable hand-off server site. The list of frequencies representing neighboring sites is referred to herein as a neighbor list or neighbor cell list. The neighbor list is ordinarily downloaded from a server site. A prior art hand-off process is described in U.S. Pat. No. 5,260,943, issued to Comroe, et al., on Nov. 9, 1993, for a TDM Hand-Off Technique Using Time Differences, the entire contents of which are hereby incorporated by reference. Here, a subscriber receives a neighbor list from a site with which it is communicating satisfactorily. This neighbor list is used to determine potential server sites for use when a hand-off becomes necessary.
Sometimes, a subscriber is unable to communicate with its server site because of gaps in coverage, and must hand off to an alternative server site to maintain an ongoing communication session. Such gaps in coverage may be caused by interference from a building, hilly terrain, and the like. One solution is to provide one or more high sites, i.e., server sites having a wide coverage area to fill in such gaps in coverage, as potential server sites in the neighbor list, to enable the subscriber to hand off to such sites. When a subscriber hands off to a new server site, it is current practice to replace its neighbor list with one downloaded from the new server site. However, a high site may be geographically far removed from the subscriber, and thus a neighbor list obtained from the high site may not have entries of sites that are within communication range of the subscriber at its current location. Consequently, a subscriber replacing its neighbor list with one downloaded from the high site may be unable to access available server sites that are within range, thereby experiencing poor performance or lost calls.
It is desirable to provide for an enhanced radio communication system in which a subscriber has adequate neighbor list information to determine potential server sites for establishing or maintaining communication. A preferred solution would provide backwards compatibility for existing system components.